Refractory composites

A third group includes silver—nickel, silver—cadmium oxide, and silver—graphite combinations. These materials are characterized by low contact resistance, some resistance to arc erosion, and excellent non sticking characteristics. They can be considered intermediate in overall properties between silver alloys and silver or copper—refractory compositions. Silver—cadmium oxide compositions, the most popular of this class, have wide appHcation in aircraft relays, motor controllers, and line starters and controls. 

Fusion-Cast Shapes. Refractory compositions are arc-melted and cast into shapes, eg, glass-tank flux blocks as large as 305 X 610 X 1219 mm. After casting and annealing, the blocks are accurately diamond ground to ensure a precise fit. 

Titanium siUcides are used in the preparation of abrasion- and heat-resistant refractories. Compositions based on mixtures of Ti Si, TiC, and diamond have been claimed to make wear-resistant cutting-tool tips (157). Titanium siUcide can be used as an electric—resistant material, in electrically conducting ceramics (158), and in pressure-sensitive elastic resistors, the electric resistance of which varies with pressure (159). 

The principal consumers of refractories are the iron and steel industries. There has been a decrease in refractories consumption that coincided with technological changes in the manufacture of steel. Steady improvements in basic oxygen furnace (BOF) practice and improvements in refractory composition and design led to improved refractory performance. More sophisticated ladle metallurgical practice has been employed, leading to improved steel quality and improved refractory performance... 

Many researchers have used glass and glass-ceramic matrices for reinforcing with high-modulus graphite fibres [1, 2], silicon carbide fibres and silicon carbide mono-filaments [3-7], Very strong, tough and refractory composites were obtained from these efforts. 

A very minor population of CAIs have ultra-refractory compositions, with enrichments in the most refractory trace elements of over a factor 1,000 times Cl chondritic composition. The most spectacular of these is OSCAR, which consists of... 

Although not as severe as fluorides, chlorides also reduce performance when they contact refractories in both the gaseous and liquid states. Chlorides also form soluble salts with most components of a refractory composition though they do not react with glassy materials. An excellent refractory material for use in chloride-containing service where temperatures do not exceed 1900°F is fused silica. This refractory performs well in exposures to chlorides because of its vitreous or glassy morphology. 

Fig. 1. The phase diagram for Al20,-Si02 showing common refractory compositions [7], Reprinted by permission of the American Ceramic Society.

Similar analyses can be made for the other alumino-silicate refractory compositions, which are summarized below. 

Rapid vapour-phase densification of refractory composites, Golecki, 1997 [145]... 

As a well-established and relatively well-understood technique I-CVI is still widely used in both the laboratory and industry. Uniform temperatures and pressures within the reactors can be easily realised in practice and make it suitable for industrial manufacturing. So far, very large reactors (up to 2500 mm in diameter) have been successfully designed and developed for densification of C/C, SiC and other refractory composites [29],... 

Figure 5.29 is reprinted from Materials Sciences and Engineering R Reports, Vol. 20,1 Golecki, Rapid vapor-phase densification of refractory composites, pp. 37-124, 1997, with permission from Elsevier. 

Fibers of refractory compositions can be drawn directly from solution (18). These fibers can be compacted for use in corrosive environments such as reinforced cement. 

Development of CVI SiC/SiC composites began in the 1980s when SEP, Amercorm, Refractory Composites and others began to develop equipment and processes for producing... 

It seems unlikely that polycrystalline oxide fibers will ever approach the use temperatures of SiC-based or C fibers (> 1500°C). Hence, it can be anticipated that even in long-term use, materials selection for refractory composites will balance the high temperature properties of non-oxide composites versus the superior enviro-thermal durability of oxide composites. 

Researchers at NASA Lewis Research Centre have used SiC monofilaments for the reinforcement of glass-ceramic matrices of refractory compositions, including strontium aluminosilicate (SAS) [82] and barium aluminosilicate (BAS) [83], with temperature capability of up to 1600°C. 

SUMMARY OF THE EIGHTH REFRACTORY COMPOSITES WORKING GROUP MEETING. 

As it was pointed out, halide may contain the element to be nitrided in a number of cases. This fact laid the basis for obtaining refractory compositions from SHS-Az systems in which only one initial combustible element is present. The second one is formed because of halide decomposition which is contained in the same initial mixture. This is rather efficient both for economical reasons (for example, boron appears more effective in the formation of halide in initial mixtures) and... 

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